Technical Field
The present disclosure relates to an anisotropic surface enhanced Raman scattering (SERS) nanoassembly of gold nanoparticles. The present disclosure further relates to an apparatus comprising the nanoassembly for detecting an analyte. Additionally, the present disclosure relates to a method for producing the nanoassembly as well as its application in a method for measuring the SERS signal of an analyte.
Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Surface enhanced Raman scattering (SERS) has become a center of interest in recent science and technology [Y. Kitahama, M. K. Hossain and Y. Ozaki, Raman, Infrared, and Near-infrared Chemical Imaging (Ed: S. Sasic, Y. Ozaki), John Wiley and Sons, Inc., Hoboken, N.J., 2010; and R. A. Halvorsen and P. J. Vikesland, Environ. Sci. Tech., 2010, 44, 7749-7755; and M. K. Hossain and Y. Ozaki, Curr. Sci., 2009, 97, 192-201; and Y. Ozaki, K. Kneipp and R. Aroca, Frontiers of surface-enhanced Raman scattering; John Wiley & Sons Ltd., Chichester, 2014; and E. C. Le Ru and P.G. Etchegoin, Principles of surface-enhanced Raman spectroscopy and related plasmonic effects, Elsevier, Amsterdam, 2009; and K. Kneipp, M. Moskovits and H. Kneipp, Surface, Enhanced Raman Scattering-Physics and applications, Springer, Heidelberg and Berlin, 2006; and R. Aroca, Surface-Enhanced Vibrational Spectroscopy, John Wiley & Sons Ltd., Chichester, 2006.—each incorporated herein by reference in its entirety]. SERS is not only simple with single molecule detection capability but also inherits the fine molecular specificity from the Raman effect of the analyte of interest [M. K. Hossain, Y. Kitahama, G. Huang, X. Han and Y. Ozaki, Anal. Bioanal. Chem., 2009, 394, 1747-1761; and S. Schlucker, Angew: Chem. Int. Ed., 2014, 53, 4756-95; and K. A. Willets, Chem. Soc. Rev., 2014, 43, 3854-3864; and M. K. Hossain, Mater. Sci. forum, 2013, 754, 143-169; and P. L. Stiles, J. A. Dieringer, N. C. Shah, R. P. VanDuyne, Annu. Rev. Anal. Chem., 2008, 1, 601-626; and G. Mcnay, D. Eustace, W. E. Smith, K. Faulds and D. Graham, Appl. Specs., 2011, 65, 825-837; and H. A. Atwater, Sci. Am., 2007, 296, 56-62; and M. K. Hossain, G. R. Willmott, P. E. Etchegoin, R. Blaikie and J. R. Tallon, Nanoscale, 2013, 5, 8945-50; and P. G. Etchegoin and E. C. Le Ru, Surface Enhanced Raman Spectroscopy (Ed.: S. Schlucker), Wiley-VCH, Weinheim, 2011; and S. E. J Bell and A. Stewart, Surface Enhanced Raman Spectroscopy (Ed.: S. Schlucker), Wiley-VCH, Weinheim, 2011; and K. A. Willets and R. P. VanDuyne, Annu. Rev. Phys. Chem., 2007, 58, 267-297.—each incorporated herein by reference in its entirety]. Since SERS demands the presence of a metallic nanostructure, the phenomenon results not only from light-molecule interactions but also from light-metal interactions. The main causative factor of dramatic SERS enhancements is now known: “the analyte” must be at “the hotsite”, which is the region of strong and localized electromagnetic (EM) field modulated by the analyte through its absorption and orientation. Two mechanisms are implicated in the SERS effect: EM and charge transfer (CT). It is widely accepted that the EM mechanism is more important, where surface plasmon resonances (SPRs) are induced at the interface or curvature by incident photons, causing an enormous increase in the EM field. The Raman signal of an analyte under such conditions will be enhanced by several orders of magnitude, typically 106-1010 fold. Noble metal nanoparticles, particularly unit dimers with a small interparticle gap, show a sharp plasmon excitation mediated EM field, leading to large signal enhancement, which facilitates single molecule detection in SERS [S. Nie and S. R. Emory, Science, 1997, 275, 1102-1106; and K. Imura, H. Okamoto, M. K. Hossain and M. Kitajima, Nano Lett., 2006, 6, 2173-2176; and G. Haran, Acc. Chem. Res., 2010, 43, 1135-1143—each incorporated herein by reference in its entirety]. Interestingly, such an EM field enhancement is strongly dependent on the incident polarization, where in-plane polarization to the interparticle axis induces the strongest enhancement [E. C. Le Ru, M. Meyer, E. Balackie and P. G. Etchegoin, J. Raman Spectros., 2008, 39, 1127-1134; and P. G. Etchegoin, C. Galloway and E. C. Le Ru, Phys. Chem. Chem. Phys., 2006, 8, 2624-2628; and. E. C. Le Ru and P. G. Etchegoin, MRS Bull, 2013, 38, 631-640; and W. R. C. Somerville, B. Auguie and E. C. Le Ru, J. Quant. Spectros. & Radia. Trans., 2013, 123, 153-168; and E. C. Le Ru, L. Schroeter and P. G. Etchegoin, Anal. Chem., 2012, 84, 5074-5079.—each incorporated herein by reference in its entirety]. Rigorous theoretical studies as well as some experimental studies have been undertaken to verify the mechanism underlying such an enhancement [F. J. Garcia-Vidal and J. B. Pendry, Phys. Rev. Lett., 1996, 77, 1163-1166; and H. Xu and M. Kall, ChemPhyChem, 2003, 4, 1001-1005.—each incorporated herein by reference in its entirety]. The polarization dependence of SERS has been investigated using silver dimers, nanorods, and coupled nanowires [M. Suzuki, W. Maekita, Y. Wada, K. Kitajima, K. Kimura, T. Fukuoka and Y. Mori, App Phys. Lett., 2006, 88, 203121-1-203121-3; and A. R. Tao and P. Yang, J. Phys. Chem. B, 2005, 109, 15687-15690; and A. G. Brolo, E. Aretander and C. J. Addison, J. Phys. Chem. B, 2005, 109, 401-405.—each incorporated herein by reference in its entirety]. The results of these investigations may throw light on the fundamental aspects of SERS mechanism(s) as well as analyte-metal interactions. However, most of the SERS studies have been performed using an ensemble system, where hotsites are not isolated but interact with each other and thus lose their inherent characteristics.
In ensemble SERS measurements, contrary to a single hotsite, many interstitials participate in SERS enhancement along with their wide variety of plasmon excitations [M. K. Hossain, Y. Kitahama, V. P. Biju, T. Kaneko, T. Itoh and Y. Ozaki, J. Phys. Chem. C, 2009, 113, 11689-11694.—incorporated herein by reference in its entirety]. However, many-particle aggregates or colloid-based nanostructures are reported to provide isotropic and inhomogeneous SPRs mediated EM field localization [M. K. Hossain, G. G. Huang, T. Kaneko and Y. Ozaki, Chem. Phys. Lett., 2009, 477, 130-134; and T. Itoh, V. Biju, M. Ishikawa, Y. Kikkawa, K. Hashimoto, A. Ikehata and Y. Ozaki, J. Chem. Phys. 2006, 124, 134708-1-134708-6.—each incorporated herein by reference in its entirety]. Furthermore, a broad SPR excitation peak, which is much different from the individual narrow excitations of isolated hotsites [L. Novotny and B. Hecht, Principles of Nano-Optics, Cambridge University Press, Cambridge, 2006.—incorporated herein by reference in its entirety]. Hence, limited-particle aggregates or nanostructures with a unique assembly are essential and indispensable to understand SERS enhancement as well as polarization dependent and polarization selective SERS characteristics. However, most of the SERS studies have considered a single dimer where the polarization effect was explained, but not the ensemble enhancement [Z. Li and H. Xu, J. Quant. Spectros. & Radia. Trans., 2007, 103, 394-401; and K. D. Alexander, M. J. Hampton, S. Zhang, A. Dhawan, H. Xu and R. A. Lopeza, J. Raman Spectrosc., 2009, 40, 2171-2175; and D. F. Zhang, Q. Zhang, L. Y. Niu, L. Jiang, P. G. Yin and L. Guo, J. Nanopar. Res., 2011, 13, 3923-3928—each incorporated herein by reference in its entirety]. Further, fixed polarization has been adopted for macroscopic samples, and hence, variations in the interstitials and SPRs have hardly been explored.
In view of the forgoing, one object of the present disclosure is to provide anisotropic gold nanoassemblies comprising nanoparticles neither in physical contact nor agglomerated but rather separated by small interparticle gaps that provide high SERS activity. Using such anisotropic gold nanoassemblies allows fine tuning of polarization dependent and polarization selective SERS measurements and background fluorescence signals with emphasis on spectroscopic measurements with reference to available active sites (i.e. localized EM fields) rather than diffraction limited imaging. A further aim of the present disclosure is a method and apparatus comprising the SERS active gold nanoassembly for measuring the surface enhanced Raman scattering (SERS) signal of an analyte and/or detecting an analyte. A further aim of the present disclosure is to provide a simple one step evaporation assisted nanoparticle assembly process to fabricate the anisotropic gold nanoassemblies.